This application claims the priority of Application No. 01118778.8, filed Aug. 9, 2001 in Europe, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method of operating a hot-chamber pressure diecasting machine in which liquid metal is delivered in shots by a plunger, which can be moved back and forth, from a casting vessel dipping into a metal bath, through the ascending duct of the casting vessel, to a mouthpiece body and a nozzle tip into a mold and is pressurized there.
The invention also relates to a hot-chamber pressure diecasting machine for implementing such a method.
Metal diecast parts are increasingly used in all technical fields where the highest possible product quality is demanded. It is known in this respect (German Patent Document DE-PS 29 22 914) to utilize path-dependent signals for controlling the pressing-in operation or signals which are dependent on the pressing-in pressure and from which conclusions can be drawn concerning the respective position of the casting plunger and thus concerning the filling ratio of the mold. From International Patent Document WO 95/33588, it is known to provide, in the end area of the mouth piece, thus just in front of the nozzle tip, a metal sensor projecting from above into the mouthpiece, in order to obtain precise actual values concerning the position of the metal front during the shot and to derive therefrom corresponding control signals for the filling of the mold and the course of the pressing-in pressure. Together with the use of highly dynamic continuous valves with very low switching times, this measure is used for obtaining better products while the casting is thin-walled.
However, care has to be taken in all cases that the air present in the mold before the shooting-in of the metal and the air situated in the ascending duct and in the mouthpiece body can escape as completely as possible in order to avoid as much as possible the formation of bubbles within the diecast part. It is known that all hot-chamber diecasting machines operate such that, after each shot, the casting plunger is moved back into its original position in which a connection opening is opened up between the metal bath tempered in the furnace and the casting cylinder in order to refill the casting cylinder first emptied during the casting operation. During this return movement of the casting plunger, a certain vacuum is generated in the ascending duct and in the mouthpiece body. Since, in addition, the mouthpiece body also slightly rises toward the nozzle tip and toward the mold, metal which still exists there after the diecasting operation flows back into the casting vessel to a level which is determined by the level of the metal bath. The ascending duct and the mouthpiece body are therefore filled with air before each shot and care must be taken that also this air, which is driven along in front of the metal front during the mold filling operation, can escape through the mold. This has the result that the mold filling operation cannot take place at a high velocity which is possible per se for the metal filling of the mold. In addition, the delivery operation of the molten metal and thus the forward movement of the casting plunger can start only when the mold, which is opened up first after the preceding shot for the removal of the workpiece, is closed again. As a result, the cycle period between each shot is extended, particularly by the casting plunger which is moving at a very low rate in the first filling phase. In addition, it is nevertheless not possible to cause all the air to escape through the venting ducts assigned to the mold, so that air voids may be formed in the cast part.
It is an object of the present invention to provide a remedy here and to suggest a method of operating a hot-chamber pressure diecasting machine and a correspondingly designed pressure diecasting machine by means of which the air can be removed from the mold and from the casting system in a relatively reliable manner.
For achieving this object, it is provided in the case of the method of the initially mentioned type according to the invention that, after each shot, while the mold is open, the casting plunger is moved from its withdrawn position, in which it opens up an inflow from the metal bath into the casting cylinder of the casting vessel, into an advanced position, in which the ascending duct and the mouthpiece body are filled with liquid metal, that the mold is then closed and only then will metal be pressed into the mold.
As a result of this measure, the time period, during which the mold is open anyhow for the removal of the workpiece, can be utilized for a segment of the delivery operation of the molten metal for the new shot. Simultaneously, it is provided that the air situated in the ascending duct and in the mouthpiece body is pressed out of the casting system, in which case it is not difficult for the air to escape because of the still open mold. After the implemented closing of the mold, the actual pressure diecasting operation can then be initiated during which only the air still present in the mold has to be pressed out of the mold through the corresponding venting ducts before the pressure onto the molten metal is increased and the pressing-in operation is completely implemented. At least the amount of air, which in the prior art has to be pressed out of the ascending duct and the mouthpiece body, that is, out of the casting system itself, during each shot, can escape in a simple manner, specifically during a time period which has to be provided anyhow for the removal of the cast part between each shot.
As a further development of the invention, the arrival of the liquid metal at the mouthpiece tip can be detected, the mold closing operation can be initiated and the casting plunger can be held until the mold is closed. As a result of this further development, in comparison to the prior art, because of the prefilling of the casting system, the actual mold filling and pressing-in operation can be carried out significantly faster and more accurately, so that it will be possible to manufacture with great efficiency high-quality products without any trapped air.
For the implementation of the new method, a hot-chamber pressure diecasting machine is suitable which is equipped with a duct ascending in the mouthpiece body from the ascending duct to the nozzle tip and with a metal sensor arranged in the area of the nozzle tip, in which case, however, the metal sensor is mounted differently than in the suggestion of International Patent Document WO 95/33588, namely on the underside of the mouthpiece body. Specifically, as a result of this further development, the metal sensor is flooded by the liquid metal which, in the prefilling phase, slowly fills up the casting system, and a sensitive control can take place. As known, mouthpiece bodies of available hot-chamber diecasting machines have a slope of approximately 5xc2x0, which can be utilized for causing the metal in this inclined mouthpiece body duct to slowly rise, as a result of the forward movement of the casting piston, until reaching the metal sensor.
A corresponding control device can now be provided which, in each case, from the point in time of the signal, which is emitted by the metal sensor and indicates that the casting system is prefilled, takes over the control of the pressing-in operation of the molten metal into the mold.
If it is providedxe2x80x94which is known per sexe2x80x94that the bath level of the molten metal in the furnace, for example, by way of a multichamber furnace, is always kept equally high, it will also be possible to indicate a casting system prefilling curve which is specific to the machine and independent of the mold. Subsequently, the casting plunger advancing and pressure admission system will operate as a function of the mold and can be operated in a known manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.